The present invention relates to a method and apparatus for distilling a mixture within a distillation column containing structured packing. More particularly, the present invention relates to such a method and apparatus in which the mixture components have close relative volatilities such as in the separation of argon from oxygen. Even more particularly the present invention relates to such a method and apparatus in which the distillation is effectuated in multiple beds of structured packing and the lowermost bed of packing has a lower height than the next higher bed of packing.
Two components may be separated from one another in a distillation column by introducing a gaseous mixture containing the two components into a bottom region of the distillation column. The introduction of the gaseous mixture initiates formation of an ascending vapor phase of the mixture. This ascending vapor phase is condensed at the top of the column to produce reflux, thereby to initiate formation of a descending liquid phase of the mixture. As the liquid phase descends it contacts the ascending vapor phase within a liquid-vapor mass transfer device such as a structured or random packing. As a result of such contact, the ascending vapor phase becomes every more concentrated in the higher volatility component while the liquid phase becomes ever more concentrated in the lower volatility component.
As the relative volatility between the components approach unity, that is the components are close boiling components, the distillation becomes more difficult. In order to accomplish such a distillation a greater height of packing is used as compared with the packing height required for distillations not involving the separation of mixtures having such close boiling components. Examples of such close boiling components are argon and oxygen, ethyl benzene and styrene and etc.
In the case of packed columns, either random or structured, the height of packing for a particular distillation can be experimentally determined. This height is based on the height equivalent to a theoretical plant or HETP. The number of theoretical plates or stages required for a particular distillation is determined in a known manner with reference to the vapor-liquid equilibrium curve of the particular mixture to be separated. The column is then packed with a sufficient amount of packing to equal to the number of theoretical plates or stages required for the distillation.
It has been found, by the inventors herein, that a factor of safety must be used in such calculations, especially in case of distilling close boiling components, because distillation column does not perform in the exact same manner as that suggested by theoretical calculations and experimental data for a particular packing. Specifically, if multiple packed beds are used and all beds are of equal height, then the performance of the lowermost bed will be the worst out of all the beds. As such, columns are taller and therefore use more packing than would initially thought to be necessary based upon theoretical calculation and experimental data.
As will be discussed, the inventors have discovered a method of conducting a distillation of close boiling components that can be effectuated by a column design that more closely mirrors theoretical calculations than prior art column designs.